Since the 1970s, the U.S. Geological Survey's National Wildlife Health Center in Wisconsin has tracked mass deaths among birds, fish and other critters, said wildlife disease specialist LeAnn White. At times the sky and the streams just turn deadly. Sometimes it's disease, sometimes pollution. Other times it's just a mystery. In the past eight months, the USGS has logged 95 mass wildlife die-offs in North America and that's probably a dramatic undercount, White said. The list includes 900 some turkey vultures that seemed to drown and starve in the Florida Keys, 4,300 ducks killed by parasites in Minnesota, 1,500 salamanders done in by a virus in Idaho, 2,000 bats that died of rabies in Texas, and the still mysterious death of 2,750 sea birds in California.

On average, 163 such events are reported to the federal government each year, according to USGS records. And there have been much larger die-offs than the 3,000 blackbirds in Arkansas. Twice in the summer of 1996, more than 100,000 ducks died of botulism in Canada. "Depending on the species, these things don't even get reported," White said.

Weather — cold and wet weather like in Arkansas New Year's Eve when the birds fell out of the sky — is often associated with mass bird deaths, ornithologists say. Pollution, parasites and disease also cause mass deaths. Some are even blaming fireworks for the blackbird deaths.

natgeo wrote:At any given time there are "at least ten billion birds in North America ... and there could be as much as 20 billion—and almost half die each year due to natural causes," said ornithologist Greg Butcher, director of bird conservation for the National Audubon Society in Washington, D.C